1. Field of the Invention
The present invention relates to the field of capture, controlling, managing, transporting and emission of currents while minimizing stray current loss. The field of the present invention further relates to the field of electron capture and subsequent emission of the charge as a field or current.
2. Background of the Art
Field emission is a phenomenon which occurs when an electric field proximate the surface of an emission material narrows a width of a potential barrier existing at the surface of the emission material. This allows a quantum tunneling effect to occur, whereby electrons cross through the potential barrier and are emitted from the material. This is as opposed to thermionic emission, whereby thermal energy within an emission material is sufficient to eject electrons from the material. Thermionic emission is a classical phenomenon, whereas field emission is a quantum mechanical phenomenon.
The field strength required to initiate field emission of electrons from the surface of a particular material depends upon that material's effective “work function.” Many materials have a positive work function and thus require a relatively intense electric field to bring about field emission. Some materials do, in fact, have a low work function, or even a negative electron affinity, and thus do not require intense fields for emission to occur. Such materials may be deposited as a thin film onto a conductor, resulting in a cathode with a relatively low threshold voltage required to produce electron emissions.
In prior art devices, it was desirable to enhance field emission of electrons by providing for a cathode geometry which focused electron emission at a single, relatively sharp point at a tip of a conical cathode (called a micro-tip cathode). These micro-tip cathodes, in conjunction with extraction grids proximate the cathodes, have been in use for years in field emission displays.
For example, U.S. Pat. No. 4,857,799 (Spindt et al.) is directed to a matrix-addressed flat panel display using field emission cathodes. The cathodes are incorporated into the display backing structure, and energize corresponding cathodoluminescent areas on a face plate. The face plate is spaced 40 microns from the cathode arrangement in the preferred embodiment, and a vacuum is provided in the space between the plate and cathodes. Spacers in the form of legs interspersed among the pixels maintain the spacing, and electrical connections for the bases of the cathodes are diffused sections through the backing structure. Spindt et al. employ a plurality of micro-tip field emission cathodes in a matrix arrangement, the tips of the cathodes aligned with apertures in an extraction grid over the cathodes. With the addition of an anode over the extraction grid, the display described as a triode (three terminal) display.
Unfortunately, micro-tips employ a structure which is difficult to manufacture, since the micro-tips have fine geometries. Unless the micro-tips have a consistent geometry throughout the display, variations in emission from tip to tip will occur, resulting in unevenness in illumination of the display. Furthermore, since manufacturing tolerances are relatively tight, such micro-tip displays are expensive to make.
For years, others have directed substantial effort toward solving the problem of creating cathodes which can be mass manufactured to tight tolerances, allowing them to perform with accuracy and precision. Another object of some of these prior art inventions was that they made use of emission materials having a relatively low effective work function so as to minimize extraction field strength.
One such effort is U.S. Pat. No. 3,947,716 (Fraser), directed to a field emission tip on which a metal adsorbent has been selectively deposited. In a vacuum, a clean field emission tip is subjected to heating pulses in the presence of an electrostatic field to create thermal field build up of a selected plane. Emission patterns from this selected plane are observed, and the process of heating the tip within the electrostatic field is repeated until emission is observed from the desired plane. The adsorbent is then evaporated onto the tip. The tip constructed by this process is selectively faceted with the emitting planar surface having a reduced work function and the non-emitting planar surface as having an increased work function. A metal adsorbent deposited on the tip so prepared results in a field emitter tip having substantially improved emission characteristics. Since emission occurs from a relatively sharp tip, emission is still somewhat inconsistent from one cathode to another. Such disadvantages become intolerable when many cathodes are employed in great numbers such as in a flat panel display for a computer.
As is evident in the above-described cathode structure, an important attribute of good cathode design is to minimize the work function of the material constituting the cathode. In fact, some substances such as alkali metals and elemental carbon in the form of diamond crystals display a low effective work function. Many inventions have been directed to finding suitable geometries for cathodes employing negative electron affinity substances as a coating for the cathode. For instance, U.S. Pat. No. 3,970,887 (Smith et al.) is directed to a microminiature field emission electron source and method of manufacturing the same wherein a single crystal semiconductor substrate is processed in accordance with known integrated microelectronic circuit techniques to produce a plurality of integral, single crystal semiconductor raised field emitter tips at desired field emission cathode sites on the surface of a substrate in a manner such that the field emitters tips are integral with the single crystal semiconductor substrate. An insulating layer and overlying conductive layer may be formed in the order named over the semiconductor substrate and provided with openings at the field emission locations to form micro-anode structures for the field emitter tip. By initially appropriately doping the semiconductor substrate to provide opposite conductivity-type regions at each of the field emission locations and appropriately forming the conductive layer, electrical isolation between the several field emission locations can be obtained. Smith et al. call for a sharply-tipped cathode.
U.S. Pat. No. 4,307,507 (Gray et al.) is directed to a method of manufacturing a field-emitter array cathode structure in which a substrate of single crystal material is selectively masked such that the unmasked areas define islands on the underlying substrate. The single crystal material under the unmasked areas is orientation-dependent etched to form an array of holes whose sides intersect at a crystal graphically sharp point.
U.S. Pat. No. 4,685,996 (Busta et al.) is also directed to a method of making a field emitter and includes an anisotropically etched single crystal silicon substrate to form at least one funnel-shaped protrusion on the substrate. The method of manufacturing disclosed in Busta et al. provides for a sharp-tipped cathode. Sharp-tipped cathodes are further described in U.S. Pat. No. 4,855,636 (Busta et al.).
Yet another sharp-tipped emission cathode is disclosed in U.S. Pat. No. 4,964,946 (Gray et al.), disclosing a process for fabricating soft-aligned field emitter arrays using a soft-leveling planarization technique, e.g. a spin-on process.
Even though many of these disclosures employ low effective work-function materials to advantage, sharp-tipped cathodes have fundamental problems when employed in a flat panel graphic display environment, as briefly mentioned above. First, they are relatively expensive to manufacture. Second, they are hard to manufacture with great consistency. That is, electron emission from sharp-tipped cathodes occurs at the tip. Therefore, the tip must be manufactured with extreme accuracy such that, in a matrix of adjacent cathodes, some cathodes do not emit electrons more efficiently than others, thereby creating an uneven visual display in other words, the manufacturing of cathodes must be made more reliable so as to minimize the problem of inconsistencies in brightness in the display along its surface.
Electron-emitting bodies may be a thermionic cathode, for example, in a vacuum tube, but may especially be a semiconductor cathode; in the latter case, various kinds of semiconductor cathodes may be used, such as NEA cathodes, field emitters and more particularly reverse junction cathodes, as described U.S. Pat. Nos. 4,303,930 and 4,370,797. Such vacuum tubes are suitable to be used as camera tubes or display tubes, but may also be used in apparatus for Auger spectroscopy, electron microscopy and electron lithography. These devices may also be provided with a photocathode, incident radiation leading to an electron current which leaves the photocathode. Such photocathodes are used in photocells, camera tubes, image converters and photomultiplier tubes. Another application of a device according to the invention resides in so-called thermionic converters, in which thermal radiation is converted into an electron current. The devices may further include to a reservoir for such an arrangement. Such a device is known from U.S. Pat. No. 1,767,437. In this case, cesium is deposited in a discharge tube by heating a dissolved mixture of cesium chloride and barium oxide so that the cesium chloride is reduced by the released barium to metallic cesium, which spreads over the interior of the discharge tube. In an embodiment shown in the patent Specification, the mixture to be heated is provided in a side tubule attached to the vacuum tube, which afterwards is sealed off from this tube. In this arrangement, a quantity of cesium is consequently introduced only once into the vacuum space. If use is made of a semiconductor cathode, this cesium will cover the emitting surface as a mono-atomic layer, after which reduction of the quantity of cesium on the emitting surface cannot or can substantially not be compensated. Such a reduction of cesium or another material reducing the electron work function at the surface is due inter alia to desorption and migration under the influence of electric fields and gives rise to degradation of the emission. The ultimate efficiency of, for example, a reverse biased junction cathode thus remains limited to 20 to 40% of the optimum value.
U.S. Pat. No. 5,600,200 (Kumar) describes a wire mesh cathode, such as a field emission cathode for use in flat panel displays comprises a layer of conductive material and a layer of amorphic diamond film, functioning as a low effective work-function material, deposited over the conductive material to form emission sites. The emission sites each contain at least two sub-regions having differing electron affinities. The cathode may be used to form a computer screen or a fluorescent light source.